1222 Volume 56, Number 9, 2002 APPLIED SPECTROSCOPY0003-7028 / 02 / 5609-1222$2.00 / 0q 2002 Society for Applied Spectroscopy

Analysis and Application of the Transmission Spectrum of aComposite Optical Waveguide

ZHI-MEI QI,* NAOKI MATSUDA,† AKIKO TAKATSU, KENJI KATO, andKIMINORI ITOHNanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba305-8565, Japan (Z.Q., N.M.); National Metrological Laboratory, National Institute of Advanced Industrial Science andTechnology, 1-1-4 Higashi, Tsukuba 305-8563, Japan (A.T., K.K.); and Graduate School of Environment and Information Sciences,Yokohama National University, Yokohama 240-8501, Japan (K.I.)

A composite optical waveguide (OWG) was fabricated by use of themasked sputtering method to deposit a tapered TiO2 � lm onto asingle-mode potassium ion-exchanged (PIE) planar waveguide. Us-ing the slab OWG spectroscopy apparatus, including a xenon lampand a charge-coupled device (CCD) spectrometer, transmissionspectra of the composite OWG with different superstrates weremeasured in the wavelength range 400 to 900 nm. The � ndings showthat the composite OWG transmission spectrum is sensitive to therefractive index of the liquid overlaid on the TiO2 � lm. The sensingmechanism is considered to be the liquid-index-induced change insurface-scattering loss for the guided mode coupled adiabaticallyfrom the PIE layer into the TiO2 � lm. The composite OWG trans-mission spectrum and its response to liquid index exhibit strongpolarization dependence due to large differences in properties be-tween the TE0 and TM0 modes in the TiO 2 � lm. The compositeOWG transmission spectroscopy as a new refractometry was ana-lyzed experimentally and theoretically.

Index Headings: Composite optical waveguide; Transmission spec-troscopy; Surface-scattering loss; Liquid-index sensing.

INTRODUCTION

Potassium ion-exchanged (PIE) glass waveguides arecheap, robust, low-loss, easy to fabricate, and convenientto couple with � bers or prisms. They have been widelyused as chemical and biological sensors based on ab-sorption,1 � uorescence,2 interference,3 waveguide-cou-pled surface plasmon resonance,4 and waveguide-basedoptoelectrochemistry.5 We used PIE planar waveguidesto construct a slab OWG spectroscopy instrument for insitu and real-time investigation of interfacial behaviors ofpigments and chromoproteins in solution samples.6,7 Sev-eral investigations into interfacial properties of dyes suchas methylene blue,8 rhodamine 6G,7 and tetra-t-butyl-phthalocyaninato copper9 reveal that the slab OWG spec-troscopy containing a PIE waveguide transducer is veryeffective for detecting adsorbed amount, adlayer orien-tation, interfacial intermediates, and products of surface-active chromophores. However, for analyses of thosedyes with weak surface activities or without surface ac-tivities, it is necessary to improve sensitivity of the slabOWG spectroscopy. In addition, the detection of tracedyes in liquid samples also requires a highly sensitivewaveguide to serve as transducer of the slab OWG spec-

Received 28 January 2002; accepted 22 April 2002.* On leave from State Key Laboratory of Transducer Technology, In-

stitute of Electronics, Chinese Academy of Sciences, Beijing 10080,China.

† Author to whom correspondence should be sent.

troscopy. From these points of view, we attempted toapply the so-called composite OWG to the slab OWGspectroscopy. A composite OWG composed of a single-mode PIE waveguide locally coated with a tapered � lmof high-index material has been demonstrated to be ableto offer a signi� cantly enhanced evanescent � eld.10–12

Moreover, the composite OWG retains most of the ad-vantages of PIE waveguides. It was found during inves-tigation of the composite OWG spectroscopy that a smallchange in refractive index of the liquid sample couldcause variation in the composite OWG transmission spec-trum. This � nding indicates that the effect of liquid sam-ple index on the evanescent absorbance has to be takeninto account as the composite OWG is used for improv-ing sensitivity of the slab OWG spectroscopy. This paperfocuses on another indication of the � nding: a direct mea-surement of the composite OWG transmission spectrumcan serve as a novel optical method for sensing liquidindex. It is well known that optical measurement of liquidindex as a powerful technique for detecting analyte con-centrations in solutions and for distinguishing similarcompounds such as alcoholic and petrolic liquids haspractical applications in many � elds. Therefore, study ofthe composite OWG transmission spectroscopy is of con-siderable importance. Compared with other OWG refrac-tometers working at a single wavelength, for example,input and output grating coupler sensors,13 resonant mir-ror sensors,14 and polarimetric and Mach–Zehnder inter-ferometers,3,11,13,15 the composite OWG transmission spec-troscopy allows evaluation of the refractive index sensi-tivity with different parameters, such as wavelength shiftand intensity attenuation, so that the measured resultsshould be highly accurate. Investigation of the compositeOWG transmission spectrum also allows for a spectro-scopic characterization of waveguide itself, which is vitalto the application of ITO � lm-coated PIE waveguides tospectroelectrochemical sensors.5,16,17 In this paper, boththe TE- and TM-polarized transmission spectra of TiO2 /PIE composite OWGs were measured and compared. Therefractive index sensitivity of the composite OWG trans-mission spectroscopy was tested with aqueous sugar so-lutions as samples. The experimental results were ex-plained rationally by combining the waveguide theorywith the surface-scattering theory of Tien.

EXPERIMENTAL

The TiO2/PIE composite OWG was prepared by � rst im-mersing soda-lime slide glass into molten KNO3 at 400 8C

APPLIED SPECTROSCOPY 1223

FIG. 1. Measured refractive index of the TiO2 � lm vs. wavelength.

FIG. 2. Schematic diagram of the TiO2 /PIE composite OWG spectros-copy instrument for sensing liquid index.

for 30 min and then depositing a tapered TiO2 � lm ontothe glass waveguide by the substrate-masked radio fre-quency (RF) sputtering technique.10–12 Using the spectro-scopic ellipsometry method, the 1-cm-long TiO2 � lm wasmeasured to have a saddlebacked thickness pro� le witha peak thickness of 26.7 nm. Figure 1 shows the wave-length dependence of the refractive index of the TiO2 � lm(each � lled circle represents an average over ten valuesof refractive index measured at a given wavelength). Theexperimental setup for the composite OWG spectroscopyis schematically shown in Fig. 2. A Te� on cell with asilicone rubber gasket is � xed over the PIE waveguidesurface. The cell has a 2-cm-long chamber that shieldsthe tapered TiO2 � lm without any contact between thegasket and the TiO2 � lm. A pair of 45–45–908 glassprisms, made from LaSF8 material with a refractive indexof 1.835 at 587.6 nm, are kept in contact with the PIEwaveguide at both ends by introduction of a couplingliquid, methylene iodide, between them. The white lightfrom a xenon lamp is passed through a multimode quartz� ber and then focused on the input prism coupler at acentral angle of about 638 with respect to the waveguidesurface normal. The output light beam is focused on oneend of another quartz � ber and then guided to the charge-coupled device (CCD) spectrometer, which has a wave-length resolution of 0.401 nm. A polarizer is � xed infront of the input prism coupler to select the TE or TMpolarization of the incident light. The transmission spec-trum of the composite OWG can be obtained directlyfrom the computer connected to the CCD spectrometer.To smooth the transmission spectrum, the output lightintensity at a given wavelength is an arithmetic mean ofnine values measured over a 3.2-nm-wide range centeredat this wavelength. When the transmission spectrum wasobserved to become stable, aqueous sugar solutions withdifferent concentrations were introduced into the cell us-ing a syringe. After each injection of the new sample, thecomposite OWG transmission spectrum was acquiredwith the computer. All the measurements were performedat room temperature (about 23 8C). Concentrations of theaqueous sugar solutions ranged from 2 to 20% (w/w)with an interval of 2%. The solution index as a linearfunction of sugar concentration increased from 1.33589

to 1.36384 given that the refractive index of the watersolvent was 1.333.18

THEORETICAL ANALYSES

The TiO2 /PIE composite OWG is a four-layer planarwaveguide with a pair of tapered velocity couplers. Sucha structure is capable of adiabatically transferring the op-tical mode from the single-mode PIE layer into the TiO2

� lm as long as a certain region of the tapered TiO2 � lmis thick enough to support the TE0 or TM0 mode at agiven wavelength.19,20 The optical mode coupled into theTiO2 � lm not only yields an enhanced evanescent � eldbut also undergoes a much larger surface-scattering lossthan that con� ned to the PIE layer. According to Tien’stheory of surface scattering for a three-layer planar wave-guide,21 the surface-scattering loss, aS, in dB per unitlength, can be expressed as the following equation. Theequation indicates that aS for the optical mode coupledinto the TiO2 � lm will change with superstrate index.

aS 5 8.686(s122 1 s13

2)K02(n 2 2 N 2)3/2(NTeff)21 (1)

where s12 and s13 are root mean square roughnesses ofthe lower and upper surfaces of the waveguiding layer,respectively. K0 is equal to 2p/l (l is wavelength in vac-uum), n is refractive index of the waveguiding layer, Nis the modal index, and Teff is the effective thickness ofthe waveguide. In the case of the white light source, thewavelength for the TE0 mode coupled into the TiO2 � lmshould be below the cutoff value that can be calculatedusing the relation

2 2 1/2[2pT (n 2 n ) ]TiO PIE2l 5 (2)cutoff 2 2 1/2 2 2 1/2arctan[n 2 n ) /(n 2 n ) ]PIE C TiO PIE2

where T is the peak thickness of the tapered TiO2 � lm,nC is superstrate index, and n and nPIE are refractiveTiO2

indices of the TiO2 � lm and the PIE layer, respectively.It can be seen from this equation that the optical modescoupled into the TiO2 � lm will shift toward longer wave-length as nC increases. Owing to a strong interaction ofthe enhanced evanescent � eld with superstrate, the nC-induced changes in both the surface-scattering loss andthe wavelength shift for the optical modes coupled intothe TiO2 � lm should be signi� cant. In other words, thecomposite OWG transmission spectrum should be verysensitive to nC. To support this inference, the followingcalculations were carried out. By use of the given OWG

1224 Volume 56, Number 9, 2002

FIG. 3. Wavelength dependence of the calculated effective index forthe TE0 mode in a four-layer waveguide. (nS and nPIE vs. wavelengthare also shown in the � gure.)

FIG. 4. Calculated lcutoff for the TE0 mode in a 26.7-nm-thick TiO2 � lmas a function of nC.

FIG. 5. Wavelength dependence of aS for the TE0 mode in a 26.7-nm-thick TiO2 � lm (a, b, c, d , e, and f correspond to nC 5 1.333, 1.340,1.347, 1.354, 1.361, and 1.368). Insert shows nC dependences of aS at633 nm and of l corresponding to aS 5 40 dB/cm.

boundary conditions to solve the Maxwell electromag-netic equations, the modal index vs. wavelength was ob-tained for the TE0 mode in a four-layer planar waveguidecomposed of a glass substrate, a PIE layer, a 26.7-nm-thick TiO2 � lm, and a water superstrate. The refractiveindex of BK7 glass as a function of wavelength, notedin Ref. 22, was used as the substrate index (nS ) in thecalculations. To simplify calculations, the PIE layer wastreated as a step-index waveguide with a PIE depth of 2mm and a refractive-index increase of Dn 5 0.05.23 Figure3 shows the calculated results. The effective index of theTE0 mode decreases from 1.6966 to 1.5111 with increas-ing wavelength from 400 to 900 nm. From Fig. 3, lcutoff

was determined to be 680.5 nm. The modal index at l ,680.5 nm is greater than the refractive index of the PIElayer, indicating that the TE0 mode is coupled into theTiO2 � lm at l , 680.5 nm and retained in the PIE layerat l . 680.5 nm. As a result, the TE0 mode in the com-posite OWG with water superstrate will suffer a largeraS at l , 680.5 nm than at l . 680.5 nm. Figure 4 showsthat the calculated lcutoff for the TE0 mode in a 26.7-nm-thick TiO2 � lm shifts linearly from 680.5 to 725.4 nm asnC increases from 1.333 to 1.367. By � rst calculating Nand Teff, then substituting the calculated values into Eq.1, aS for the TE0 mode in the 26.7-nm-thick TiO2 � lmwas obtained as a function of wavelength. Here, the val-ues of s12 and s13 in Eq. 1 were given to be 0.5 and 1.3nm, as described in the former paper.23 The six curves,a, b, c, d, e, and f , shown in Fig. 5 correspond to nC 51.333, 1.340, 1.347, 1.354, 1.361, and 1.368, respective-ly. It can be seen that aS decreases with wavelength inthe case of a given nC and increases with nC at a speci� cwavelength. Moreover, the wavelength corresponding toa constant aS increases with nC. The insert in Fig. 5 showsthat both the aS at 633 nm and the wavelength corre-sponding to aS 5 40 dB/cm are linearly dependent on nC.The values of DaS /DnC and Dl/DnC are determined to be499.62 dB/cm and 775.78 nm, respectively, which can beregarded as the theoretical refractive index sensitivitiesof the composite OWG transmission spectroscopy in thecase of a 1-cm-long interaction path. With these calcu-lations, the experimental results described below becomeeasy to understand. It should be noted that quantitative

differences between the measured and calculated scatteringlosses were mainly attributed to the fact that the taperedTiO2 � lm of the composite OWG used in the experimentcontained a 26.7-nm-thick region much shorter than the1-cm-long interaction path involved in calculations.

RESULTS AND DISCUSSION

Figure 6 shows the TiO2 /PIE composite OWG trans-mission spectra measured with the TE-polarized incidentlight. It is obvious that different superstrate indices resultin different spectra. With air superstrate the spectrum hasa wavelength range of 490 to 900 nm. When 2% aqueoussugar solution was introduced into the cell to cover theTiO2 � lm, the output light disappeared at l , 550 nm,indicating that the guided light at l , 550 nm was cou-pled into the TiO2 � lm and then scattered completely dueto a great aS. The calculated aS at l , 550 nm is greaterthan 150 dB/cm. The light intensity at l . 750 nm, how-ever, was almost unchanged, implying that the guidedlight at l . 750 nm was con� ned to the low-loss, insen-sitive PIE layer. It can be seen from the transmission

APPLIED SPECTROSCOPY 1225

FIG. 6. Measured TE-polarized transmission spectra of the compositeOWG in the case of different superstrates. FIG. 7. Loss spectra of the composite waveguide derived from Fig. 6.

Insert shows a at 633 nm and l corresponding to a 5 9 dB vs. nC.

FIG. 8. Comparison between the TE- and TM-polarized transmissionspectra of the composite OWGs. (a) TM and (b) TE for the 26.7 nmTiO2 � lm; (c) TM and (d ) TE for the 58.6 nm TiO2 � lm.

spectra that the guided light only in the range 570 to 720nm has the ability to respond to changes in liquid index.The output light attenuation vs. wavelength in this range(i.e., the loss spectrum) is shown in Fig. 7. Here, theattenuation, a, is de� ned as a 5 10log (IA /IS ); IA and IS

are the output light intensities at a given wavelength inthe case of air and liquid sample superstrates. Similar toFig. 5, a decreases with wavelength for a given sugarconcentration and increases with sugar concentration at aspeci� c wavelength. In the case of a constant a, wave-length increases with sugar concentration. The insert inFig. 7 shows that both the wavelength corresponding toa 5 9 dB and the a at 633 nm are a linear function ofnC. In the sense of two linear relationships, the experi-mental results are in accordance with the calculated data.On the basis of the calculated results shown in Fig. 4,extension of the loss spectra toward longer wavelengthwith increases in nC should be attributed to nC-inducedincreases in lcutoff. From the insert in Fig. 7, the value ofDl/DnC is determined to be 842 nm. The CCD spectrom-eter has a resolution of 0.401 nm, which means that inprinciple, the TE-polarized transmission spectroscopy ofthe composite OWG can respond to a change of 5 3 1024

in liquid index by evaluating the wavelength shift. It hasbeen found in the experiment that such a resolution isaffected by the poor stability of the prism coupling. Useof the single-mode � ber to couple with the PIE wave-guide is expected to improve the sensor resolution. It wasalso derived from the insert in Fig. 7 that Da/DnC wasequal to 112.2 dB at l 5 633 nm. The slab OWG spec-troscopy instrument used can accurately detect a changeof 0.1 dB in signal intensity. Therefore, by examining thesignal attenuation at a given wavelength, the TE-polar-ized spectroscopy of the composite OWG has a resolutionof DnC 5 8.91 3 1024. In other words, with a 633 nmHeNe laser source, the composite OWG can respond tosuch an DnC.10

The TM-polarized transmission spectra of the sameTiO2 /PIE composite OWG were also examined. Asshown in Fig. 8, spectrum a, with air superstrate the TM-polarized spectrum ranged from 430 to 900 nm. Substi-tution of water for air superstrate almost did not cause

changes in the TM-polarized spectrum, indicating thatwith the TM-polarized incident light the composite OWGunder investigation was insensitive to liquid index. Thisis attributed to the fact that the tapered TiO2 � lm with amaximum thickness of 26.7 nm is too thin to support theTM0 mode in the range 400 to 900 nm. The TM-polarizedtransmission spectrum of the composite OWG was alsofound to be extremely similar to that for the bare PIEwaveguide, indicating again that the TM-polarized lightin the composite OWG was well con� ned to the PIElayer. Compared with the TM-polarized spectrum for airsuperstrate, the output light in the range 430 to 490 nmdisappeared in the TE-polarized spectrum (Fig. 8, spec-trum b). It revealed that the TE0 modes in this range werecoupled into the tapered TiO2 � lm and then scattered en-tirely. Calculations show that with air superstrate lcutoff is524.34 nm and aS is greater than 180 dB/cm at l , 490nm for the TE0 mode in a 26.7-nm-thick TiO2 � lm. Inorder to investigate the refractive-index sensitivity of theTM-polarized spectroscopy of the composite OWG, themaximum thickness of the tapered TiO2 � lm was in-creased from 26.7 to 58.6 nm. According to calculations,

1226 Volume 56, Number 9, 2002

FIG. 9. Measured TM-polarized transmission spectra of the compositeOWG in the case of different superstrates.

FIG. 10. a at 633 nm and l corresponding to a 5 9 dB as a functionof nC determined from Fig. 9.

with water superstrate a 58.6-nm-thick TiO2 � lm can sup-port the TM0 mode at l , 700 nm so that the TM-po-larization spectrum of such a composite OWG should becapable of response to liquid index. Figure 8, spectra cand d, show the TM- and TE-polarized spectra of thecomposite OWG in the case of air superstrate. The signaloccurred in the wavelength range 480 to 900 nm in theTM-polarized spectrum and disappeared at wavelengthsbelow 800 nm in the TE-polarized spectrum. Therefore,for the liquid-index-sensing application, this TiO2 /PIEcomposite OWG can work only at the TM-polarizedmode. Figure 9 shows the TM-polarized spectra of thecomposite OWG with air and various liquid superstrates.The superstrates corresponding to the curves a, b, c, d,e, f, and g are methanol (1.329), water (1.333), threesugar solutions (6, 10, and 14%), propanol (1.377), andethanol (1.363), respectively. Figure 10 shows both a at633 nm and l corresponding to a 5 9 dB as a functionof nC. Similar to the case of the TE-polarized spectros-copy (insert in Fig. 7), a and l are dependent almostlinearly on nC in the range 1.329 to 1.377. From Fig. 10,Dn corresponding to Dl 5 0.5 nm and Da 5 0.1 dB weredetermined to be 5.7 3 1024 and 8.3 3 1024, respectively.

The description above elucidates that the compositeOWG transmission spectroscopy can respond to changeson the order of 1024 in refractive index with either TE-or TM-polarized incident light. It was reported that theexperimental resolution was Dn ; 1024 for the resonantmirror sensor,14 Dn 5 5 3 1025 for the grating couplersensor,13 and Dn ; 5 3 1028 for the Mach–Zehnder in-terferometer.15 Compared with these OWG sensors, theresolution of the composite OWG transmission spectros-copy is not so ideal. However, the composite OWG trans-mission spectroscopy has some advantages, for example,simple fabrication of the composite OWG, easy operationof the instrument, and accurate evaluation of analyte in-dex with different parameters. According to this study, itis dif� cult to make the TE- and TM-polarized transmis-sion spectra of a given TiO2 /PIE composite OWG si-multaneously sensitive to liquid index because the TE0

and TM0 modes in the TiO2 � lm have signi� cantly dif-ferent properties. In the meantime, the study also suggeststhat the TM-polarized incident light should be more suit-

able for spectroelectrochemical sensors than the TE-po-larized one.17 This is because for spectroelectrochemicalsensors based on waveguides coated with the ITO � lmelectrodes, the TE-polarized light is easy to couple intothe ITO � lm and then suffers a large surface-scatteringloss compared with the TM-polarized one. It is a fact thata large surface-scattering loss often makes OWG sensorsincapable of working effectively.

CONCLUSION

The composite OWG transmission spectroscopy as anew refractometry was investigated with experimentalmeasurements and theoretical calculations. The � ndingsindicate that the composite OWG transmission spectrumand its response to liquid index are highly dependent onthe polarization state of the incident light and on the max-imum thickness of the tapered TiO2 � lm. The compositeOWG transmission spectroscopy has been demonstratedto have a detection limit of Dn ; 5 3 1024 for either TE-or TM-polarized incident light. Owing to the dependenceof surface-scattering loss on the adlayer thickness, thecomposite OWG transmission spectroscopy should havethe ability to respond to surface reactions.

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